While development of oil wells and gas wells is expanding on a global scale after the first oil shocks, increased demand for energy in developing countries increasingly forces deepening of oil wells and gas wells and the drilling of wells in a sour gas environment with further severe corrosiveness.
With such increased severity in the oil well and gas well environments, for example, various Ni base alloys, higher in strength than ever before and excellent in corrosion resistance, as shown in the Patent Documents 1 and 2, and further a super austenitic stainless steel as shown in the Patent Document 3, have been developed and practically used.
However, economic globalization, such as corporate marriage or reorganization which has rapidly progressed on a world scale through the termination of the cold war between the East and the West, the integration of the EU, or the like intensifies the price competition among companies. Consequently, in the development of oil wells and gas wells, higher efficiency and lower cost are in demand in addition to ensuring safety.
Increased productivity of oil or gas can be attained by using large diameter pipes. Moreover, the further use of strong material enables reduction in the wall thickness of the pipes, resulting in saving of material cost. Therefore, as a steel stock for pipes used in oil wells and gas wells, a material cost saving and having a higher strength than ever before is requested. The enlargement of the diameter of the pipes is also important.
On the other hand, in the development of oil wells and gas wells, reduced costs can be attained by using inexpensive material which has sufficient strength and corrosion resistance.
The Patent Document 4 thus discloses a “high Cr-high Ni alloy, excellent in stress corrosion cracking resistance”, which is enhanced in economical property by reducing the Mo content in alloys which contain, by weight %, 20 to 35% of Cr and 25 to 50% of Ni.
If piercing and rolling by a piercer can be adapted, pipe stocks for large diameter pipes or sufficiently long pipes can be efficiently manufactured at a low cost on an industrial scale.
The Patent Document 5 therefor discloses a “method for piercing a seamless tube of hard-to-work material with piercer”, which is intended to provide a manufacturing method of seamless pipes, capable of manufacturing a pipe stock for seamless pipes by a piercer without causing pipe inside surface defects resulting from overheating.
Further, the Non-Patent Document 1 discloses a technique capable of performing rolling, in the piercing and rolling of high Cr-high Ni alloys, without causing inside surface scabs or two-piece cracks by increasing the roll cross angle and the roll feed angle.
Patent Document 1: U.S. Pat. No. 4,168,188,
Patent Document 2: U.S. Pat. No. 4,245,698,
Patent Document 3: International Patent Publication Pamphlet No. WO 03/044239,
Patent Document 4: Japanese Laid-Open Patent Publication No. 11-302801,
Patent Document 5: Japanese Laid-Open Patent Publication No. 2000-301212,
Non-Patent Document 1: Tomio YAMAKAWA and Chihiro HAYASHI: CAMP-ISIJ, Vol. 6 (1993), 364.